It is general knowledge in the field that for centrifuges having a centrifugal bowl or rotor, with peripheral discharge nozzles, it is necessary to maintain a feed or influent rate equivalent to, or greater than, the discharge rate of the nozzles. If the feed rate is less than the nozzle discharge rate, the centrifugal bowl will eventually be evacuated of process material. This is an extremely hazardous condition in that any imbalance, such as that caused by plugged nozzles or uneven solids build-up in the bowl, will be accentuated by the high centrifugal forces. The imbalance condition under these circumstances can cause major equipment damage or even personal injury if the centrifuge should come apart.
A number of safety measures have been employed to avoid the potential hazards caused by driving an evacuated centrifugal bowl. These measures usually take the form of an emergency flush system. This type system uses a flow sensing device in the feed and/or effluent line. When the flow sensing device measures a low flow rate, or loss of flow, it triggers an emergency flush system for flooding the bowl. The flush system is typically (1) a pressurized line such as a water supply line, (2) a head tank containing flush liquid, or (3) a head tank containing clarified process fluid (centrifuge effluent).
Another emergency system has a water brake for emergency shut-down. This type of emergency measure uses a jacket or channellike structure which surrounds the centrifugal bowl or rotor. If an out-of-balance situation occurs, the jacket is flooded with water or some other liquid. The water acts as a brake to stop the bowl quickly. This method is usually incorporated with one of the above bowl-flooding systems since the water-brake does not counteract an imbalance condition.
The foregoing systems are disadvantageous for a number of reasons. For the first system, a large supply of pressurized water must be available to maintain a "flooded-bowl" condition while the bowl is rotating. If the centrifuge is shut-down due to loss in feed, the flush water must maintain the flooded-bowl condition until the bowl has ceased to rotate. This typically takes several minutes. Without an external means of braking (such as the water brake), the rotating bowl may take 10,20,30 minutes or more to stop.
The systems using a head tank would also require a large amount of fluid as stated above. For example, a typical industrial centrifuge with a nozzle discharge rate of 35 gpm has a deceleration time of thirty minutes, unbraked. The flush system would require a head tank of 1000 gallons to satisfy the nozzle discharge rate during shut-down.
Each of the above systems also requires some type of flow monitoring device. This device is needed to detect the loss of feed which will result in insufficient flow to satisfy the nozzles. To operate the solenoid valve for the flush liquid, an appropriate control system must be installed between the valve and the sensing device.
For industrial plant applications, the disadvantages of the foregoing systems are:
1. Floor space and overhead are needed for a head tank installation; or a pressurized, high volume fluid supply line is needed in lieu of the head tank. PA0 2. A large volume of flush liquid is needed for shut-down. PA0 3. A reliable flow sensing device and control system are needed.
In addition to the facility requirements, systems using these prior measures have inherent operational problems. Systems using pressurized water for flush liquid may lose fluid pressure or volume. For systems using centrifuge effluent for flush liquid, solids settling in pipes or thixotropic fluids may cause pipe blockage--preventing the emergency flush system from functioning. In the control system, flow sensing devices may foul, falsely giving a "no-flow" condition. Or worse, the sensing device may read full flow while actually there is no flow. Solenoid valves may malfunction due to mechanical or control problems. For pneumatic controls, loss of air pressure may mean loss of system control.
Since automatic flush control valves must open on power loss, there is usually a manual valve that can turn off the flush system when the equipment is not operating, or when power is off. Human error would be another factor in this case. Failure to reopen the manual valve on the flush line can be disasterous if an emergency should occur after the centrifuge is restarted.
For some field applications, such as those found in the oil drilling industry, the necessary facilities, namely, tank space, pressurized supply water, and large volumes of flush liquid, are often not readily available. Under these circumstances, a different emergency system is needed for safe centrifuge operation.